std::partition_point
Defined in header <algorithm>
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(1) | ||
template< class ForwardIt, class UnaryPredicate > ForwardIt partition_point( ForwardIt first, ForwardIt last, UnaryPredicate p ); |
(since C++11) (until C++20) |
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template< class ForwardIt, class UnaryPredicate > constexpr ForwardIt partition_point( ForwardIt first, ForwardIt last, UnaryPredicate p ); |
(since C++20) | |
Examines the partitioned (as if by std::partition) range [first, last)
and locates the end of the first partition, that is, the first element that does not satisfy p
or last
if all elements satisfy p
.
Parameters
first, last | - | the partitioned range of elements to examine |
p | - | unary predicate which returns true for the elements found in the beginning of the range. The expression p(v) must be convertible to bool for every argument |
Type requirements | ||
-ForwardIt must meet the requirements of LegacyForwardIterator.
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-UnaryPredicate must meet the requirements of Predicate.
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Return value
The iterator past the end of the first partition within [first, last)
or last
if all elements satisfy p
.
Complexity
Given N = std::distance(first, last), performs O(log N) applications of the predicate p
.
However, for non-LegacyRandomAccessIterators, the number of iterator increments is O(N).
Notes
This algorithm is a more general form of std::lower_bound, which can be expressed in terms of std::partition_point
with the predicate [&](auto const& e) { return e < value; });.
Example
#include <algorithm> #include <array> #include <iostream> #include <iterator> int main() { std::array<int, 9> v = { 1, 2, 3, 4, 5, 6, 7, 8, 9 }; auto is_even = [](int i){ return i % 2 == 0; }; std::partition(v.begin(), v.end(), is_even); auto p = std::partition_point(v.begin(), v.end(), is_even); std::cout << "Before partition:\n "; std::copy(v.begin(), p, std::ostream_iterator<int>(std::cout, " ")); std::cout << "\nAfter partition:\n "; std::copy(p, v.end(), std::ostream_iterator<int>(std::cout, " ")); }
Output:
Before partition: 8 2 6 4 After partition: 5 3 7 1 9
See also
(C++11) |
checks whether a range is sorted into ascending order (function template) |
returns an iterator to the first element not less than the given value (function template) |